We report the fabrication and characterization of a fiber Bragg grating (FBG) with 870 nm resonance wavelength in a
single-mode TOPAS microstructured polymer optical fiber (mPOF). The grating has been UV-written with the phasemask
technique using a 325 nm HeCd laser. The static tensile strain sensitivity has been measured as 0.64 pm/μstrain,
and the temperature sensitivity was -60 pm/°C. This is the first 870nm FBG and the first demonstration of a negative
temperature response for the TOPAS FBG, for which earlier results have indicated a positive temperature response. The
relatively low material loss of the fiber at this wavelength compared to that at longer wavelengths will considerably
enhance the potential utility of the TOPAS FBG.